=head1 NAME
X
perlsyn - Perl syntax
=head1 DESCRIPTION
A Perl program consists of a sequence of declarations and statements
which run from the top to the bottom. Loops, subroutines and other
control structures allow you to jump around within the code.
Perl is a B language, you can format and indent it however
you like. Whitespace mostly serves to separate tokens, unlike
languages like Python where it is an important part of the syntax.
Many of Perl's syntactic elements are B. Rather than
requiring you to put parentheses around every function call and
declare every variable, you can often leave such explicit elements off
and Perl will figure out what you meant. This is known as B, abbreviated B. It allows programmers to be B and to
code in a style with which they are comfortable.
Perl B and concepts from many languages: awk, sed, C,
Bourne Shell, Smalltalk, Lisp and even English. Other
languages have borrowed syntax from Perl, particularly its regular
expression extensions. So if you have programmed in another language
you will see familiar pieces in Perl. They often work the same, but
see L for information about how they differ.
=head2 Declarations
X X X X
The only things you need to declare in Perl are report formats and
subroutines (and sometimes not even subroutines). A variable holds
the undefined value (C) until it has been assigned a defined
value, which is anything other than C. When used as a number,
C is treated as C<0>; when used as a string, it is treated as
the empty string, C; and when used as a reference that isn't being
assigned to, it is treated as an error. If you enable warnings,
you'll be notified of an uninitialized value whenever you treat
C as a string or a number. Well, usually. Boolean contexts,
such as:
my $a;
if ($a) {}
are exempt from warnings (because they care about truth rather than
definedness). Operators such as C, C, C,
C, and C, that operate on undefined left values such as:
my $a;
$a++;
are also always exempt from such warnings.
A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements--declarations all
take effect at compile time. Typically all the declarations are put at
the beginning or the end of the script. However, if you're using
lexically-scoped private variables created with C, you'll
have to make sure
your format or subroutine definition is within the same block scope
as the my if you expect to be able to access those private variables.
Declaring a subroutine allows a subroutine name to be used as if it were a
list operator from that point forward in the program. You can declare a
subroutine without defining it by saying C, thus:
X
sub myname;
$me = myname $0 or die "can't get myname";
Note that myname() functions as a list operator, not as a unary operator;
so be careful to use C instead of C in this case. However, if
you were to declare the subroutine as C, then
C would function as a unary operator, so either C or
C would work.
Subroutines declarations can also be loaded up with the C statement
or both loaded and imported into your namespace with a C statement.
See L for details on this.
A statement sequence may contain declarations of lexically-scoped
variables, but apart from declaring a variable name, the declaration acts
like an ordinary statement, and is elaborated within the sequence of
statements as if it were an ordinary statement. That means it actually
has both compile-time and run-time effects.
=head2 Comments
X X
Text from a C character until the end of the line is a comment,
and is ignored. Exceptions include C inside a string or regular
expression.
=head2 Simple Statements
X X X X
The only kind of simple statement is an expression evaluated for its
side effects. Every simple statement must be terminated with a
semicolon, unless it is the final statement in a block, in which case
the semicolon is optional. (A semicolon is still encouraged if the
block takes up more than one line, because you may eventually add
another line.) Note that there are some operators like C and
C that look like compound statements, but aren't (they're just
TERMs in an expression), and thus need an explicit termination if used
as the last item in a statement.
=head2 Truth and Falsehood
X X X X X X X X<0>
The number 0, the strings C and C, the empty list C, and
C are all false in a boolean context. All other values are true.
Negation of a true value by C or C returns a special false value.
When evaluated as a string it is treated as C, but as a number, it
is treated as 0.
=head2 Statement Modifiers
X X X X X
X X X X
Any simple statement may optionally be followed by a I modifier,
just before the terminating semicolon (or block ending). The possible
modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
when EXPR
for LIST
foreach LIST
The C following the modifier is referred to as the "condition".
Its truth or falsehood determines how the modifier will behave.
C executes the statement once I and only if the condition is
true. C is the opposite, it executes the statement I
the condition is true (i.e., if the condition is false).
print "Basset hounds got long ears" if length $ear >= 10;
go_outside() and play() unless $is_raining;
C executes the statement I C smart matches C, and
then either Cs out if it's enclosed in a C scope or skips
to the C element when it lies directly inside a C loop.
See also L"Switch statements">.
given ($something) {
$abc = 1 when /^abc/;
$just_a = 1 when /^a/;
$other = 1;
}
for (@names) {
admin($_) when [ qw/Alice Bob/ ];
regular($_) when [ qw/Chris David Ellen/ ];
}
The C modifier is an iterator: it executes the statement once
for each item in the LIST (with C aliased to each item in turn).
print "Hello $_!\n" foreach qw(world Dolly nurse);
C repeats the statement I the condition is true.
C does the opposite, it repeats the statement I the
condition is true (or while the condition is false):
# Both of these count from 0 to 10.
print $i++ while $i <= 10;
print $j++ until $j > 10;
The C and C modifiers have the usual "C loop"
semantics (conditional evaluated first), except when applied to a
C-BLOCK (or to the deprecated C-SUBROUTINE statement), in
which case the block executes once before the conditional is
evaluated. This is so that you can write loops like:
do {
$line = ;
...
} until $line eq ".\n";
See L. Note also that the loop control statements described
later will I work in this construct, because modifiers don't take
loop labels. Sorry. You can always put another block inside of it
(for C) or around it (for C) to do that sort of thing.
For C, just double the braces:
X X X
do {{
next if $x == $y;
# do something here
}} until $x++ > $z;
For C, you have to be more elaborate:
X
LOOP: {
do {
last if $x = $y**2;
# do something here
} while $x++ <= $z;
}
B The behaviour of a C statement modified with a statement
modifier conditional or loop construct (e.g. C) is
B. The value of the C variable may be C, any
previously assigned value, or possibly anything else. Don't rely on
it. Future versions of perl might do something different from the
version of perl you try it out on. Here be dragons.
X
=head2 Compound Statements
X X X X X
X X X X X X X X X
In Perl, a sequence of statements that defines a scope is called a block.
Sometimes a block is delimited by the file containing it (in the case
of a required file, or the program as a whole), and sometimes a block
is delimited by the extent of a string (in the case of an eval).
But generally, a block is delimited by curly brackets, also known as braces.
We will call this syntactic construct a BLOCK.
The following compound statements may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
unless (EXPR) BLOCK
unless (EXPR) BLOCK else BLOCK
unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL until (EXPR) BLOCK
LABEL until (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK
LABEL foreach VAR (LIST) BLOCK continue BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
not statements. This means that the curly brackets are I--no
dangling statements allowed. If you want to write conditionals without
curly brackets there are several other ways to do it. The following
all do the same thing:
if (!open(FOO)) { die "Can't open $FOO: $!"; }
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
# a bit exotic, that last one
The C statement is straightforward. Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
C an C goes with. If you use C in place of C,
the sense of the test is reversed. Like C, C can be followed
by C. C can even be followed by one or more C
statements, though you may want to think twice before using that particular
language construct, as everyone reading your code will have to think at least
twice before they can understand what's going on.
The C statement executes the block as long as the expression is
L.
The C statement executes the block as long as the expression is
false.
The LABEL is optional, and if present, consists of an identifier followed
by a colon. The LABEL identifies the loop for the loop control
statements C, C, and C.
If the LABEL is omitted, the loop control statement
refers to the innermost enclosing loop. This may include dynamically
looking back your call-stack at run time to find the LABEL. Such
desperate behavior triggers a warning if you use the C
pragma or the B flag.
If there is a C BLOCK, it is always executed just before the
conditional is about to be evaluated again. Thus it can be used to
increment a loop variable, even when the loop has been continued via
the C statement.
Extension modules can also hook into the Perl parser to define new
kinds of compound statement. These are introduced by a keyword which
the extension recognizes, and the syntax following the keyword is
defined entirely by the extension. If you are an implementor, see
L for the mechanism. If you are using such
a module, see the module's documentation for details of the syntax that
it defines.
=head2 Loop Control
X X X X X X
The C command starts the next iteration of the loop:
LINE: while () {
next LINE if /^#/; # discard comments
...
}
The C command immediately exits the loop in question. The
C block, if any, is not executed:
LINE: while () {
last LINE if /^$/; # exit when done with header
...
}
The C command restarts the loop block without evaluating the
conditional again. The C block, if any, is I executed.
This command is normally used by programs that want to lie to themselves
about what was just input.
For example, when processing a file like F.
If your input lines might end in backslashes to indicate continuation, you
want to skip ahead and get the next record.
while (<>) {
chomp;
if (s/\\$//) {
$_ .= <>;
redo unless eof();
}
# now process $_
}
which is Perl short-hand for the more explicitly written version:
LINE: while (defined($line = )) {
chomp($line);
if ($line =~ s/\\$//) {
$line .= ;
redo LINE unless eof(); # not eof(ARGV)!
}
# now process $line
}
Note that if there were a C block on the above code, it would
get executed only on lines discarded by the regex (since redo skips the
continue block). A continue block is often used to reset line counters
or C one-time matches:
# inspired by :1,$g/fred/s//WILMA/
while (<>) {
m?(fred)? && s//WILMA $1 WILMA/;
m?(barney)? && s//BETTY $1 BETTY/;
m?(homer)? && s//MARGE $1 MARGE/;
} continue {
print "$ARGV $.: $_";
close ARGV if eof; # reset $.
reset if eof; # reset ?pat?
}
If the word C is replaced by the word C, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.
The loop control statements don't work in an C or C, since
they aren't loops. You can double the braces to make them such, though.
if (/pattern/) {{
last if /fred/;
next if /barney/; # same effect as "last", but doesn't document as well
# do something here
}}
This is caused by the fact that a block by itself acts as a loop that
executes once, see L.
The form C, available in Perl 4, is no longer
available. Replace any occurrence of C by C.
=head2 For Loops
X X
Perl's C-style C loop works like the corresponding C loop;
that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
There is one minor difference: if variables are declared with C
in the initialization section of the C, the lexical scope of
those variables is exactly the C loop (the body of the loop
and the control sections).
X
Besides the normal array index looping, C can lend itself
to many other interesting applications. Here's one that avoids the
problem you get into if you explicitly test for end-of-file on
an interactive file descriptor causing your program to appear to
hang.
X X X
$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
for ( prompt(); ; prompt() ) {
# do something
}
Using C (or the operator form, C<< >>) as the
conditional of a C loop is shorthand for the following. This
behaviour is the same as a C loop conditional.
X X<< <> >>
for ( prompt(); defined( $_ = ); prompt() ) {
# do something
}
=head2 Foreach Loops
X X
The C loop iterates over a normal list value and sets the
variable VAR to be each element of the list in turn. If the variable
is preceded with the keyword C, then it is lexically scoped, and
is therefore visible only within the loop. Otherwise, the variable is
implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with C, it uses
that variable instead of the global one, but it's still localized to
the loop. This implicit localization occurs I in a C
loop.
X X
The C keyword is actually a synonym for the C keyword, so
you can use C for readability or C for brevity. (Or because
the Bourne shell is more familiar to you than I, so writing C
comes more naturally.) If VAR is omitted, C is set to each value.
X
If any element of LIST is an lvalue, you can modify it by modifying
VAR inside the loop. Conversely, if any element of LIST is NOT an
lvalue, any attempt to modify that element will fail. In other words,
the C loop index variable is an implicit alias for each item
in the list that you're looping over.
X
If any part of LIST is an array, C will get very confused if
you add or remove elements within the loop body, for example with
C. So don't do that.
X
C probably won't do what you expect if VAR is a tied or other
special variable. Don't do that either.
Examples:
for (@ary) { s/foo/bar/ }
for my $elem (@elements) {
$elem *= 2;
}
for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
print $count, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
print "Item: $item\n";
}
Here's how a C programmer might code up a particular algorithm in Perl:
for (my $i = 0; $i < @ary1; $i++) {
for (my $j = 0; $j < @ary2; $j++) {
if ($ary1[$i] > $ary2[$j]) {
last; # can't go to outer :-(
}
$ary1[$i] += $ary2[$j];
}
# this is where that last takes me
}
Whereas here's how a Perl programmer more comfortable with the idiom might
do it:
OUTER: for my $wid (@ary1) {
INNER: for my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
}
See how much easier this is? It's cleaner, safer, and faster. It's
cleaner because it's less noisy. It's safer because if code gets added
between the inner and outer loops later on, the new code won't be
accidentally executed. The C explicitly iterates the other loop
rather than merely terminating the inner one. And it's faster because
Perl executes a C statement more rapidly than it would the
equivalent C loop.
=head2 Basic BLOCKs
X
A BLOCK by itself (labeled or not) is semantically equivalent to a
loop that executes once. Thus you can use any of the loop control
statements in it to leave or restart the block. (Note that this is
I true in C, C, or contrary to popular belief
C blocks, which do I count as loops.) The C
block is optional.
The BLOCK construct can be used to emulate case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
Such constructs are quite frequently used, because older versions
of Perl had no official C statement.
=head2 Switch statements
X X X X X
Starting from Perl 5.10, you can say
use feature "switch";
which enables a switch feature that is closely based on the
Perl 6 proposal.
The keywords C and C are analogous
to C and C in other languages, so the code
above could be written as
given($_) {
when (/^abc/) { $abc = 1; }
when (/^def/) { $def = 1; }
when (/^xyz/) { $xyz = 1; }
default { $nothing = 1; }
}
This construct is very flexible and powerful. For example:
use feature ":5.10";
given($foo) {
when (undef) {
say '$foo is undefined';
}
when ("foo") {
say '$foo is the string "foo"';
}
when ([1,3,5,7,9]) {
say '$foo is an odd digit';
continue; # Fall through
}
when ($_ < 100) {
say '$foo is numerically less than 100';
}
when (\&complicated_check) {
say 'a complicated check for $foo is true';
}
default {
die q(I don't know what to do with $foo);
}
}
C will assign the value of EXPR to C
within the lexical scope of the block, so it's similar to
do { my $_ = EXPR; ... }
except that the block is automatically broken out of by a
successful C or an explicit C.
Most of the power comes from implicit smart matching:
when($foo)
is exactly equivalent to
when($_ ~~ $foo)
Most of the time, C is treated as an implicit smart match of
C, i.e. C. (See L"Smart matching in detail"> for more
information on smart matching.) But when EXPR is one of the below
exceptional cases, it is used directly as a boolean:
=over 4
=item *
a subroutine or method call
=item *
a regular expression match, i.e. C or C,
or a negated regular expression match (C or C).
=item *
a comparison such as C 10> or C
(or of course C)
=item *
C, C, or C
=item *
a negated expression C or C, or a logical
exclusive-or C.
=item *
a filetest operator, with the exception of C, C, C, and C,
that return numerical values, not boolean ones.
=item *
the C and C flip-flop operators.
=back
In those cases the value of EXPR is used directly as a boolean.
Furthermore, Perl inspects the operands of the binary boolean operators to
decide whether to use smart matching for each one by applying the above test to
the operands:
=over 4
=item *
If EXPR is C or C, the test
is applied recursively to both operands. If I
operands pass the test, then the expression is treated
as boolean; otherwise, smart matching is used.
=item *
If EXPR is C, C or C, the test
is applied recursively to the first operand (which may be a
higher-precedence AND operator, for example). If the first operand
is to use smart matching, then both operands will do so; if it is
not, then the second argument will not be either.
=back
These rules look complicated, but usually they will do what
you want. For example:
when (/^\d+$/ && $_ < 75) { ... }
will be treated as a boolean match because the rules say both a regex match and
an explicit test on $_ will be treated as boolean.
Also:
when ([qw(foo bar)] && /baz/) { ... }
will use smart matching because only I of the operands is a boolean; the
other uses smart matching, and that wins.
Further:
when ([qw(foo bar)] || /^baz/) { ... }
will use smart matching (only the first operand is considered), whereas
when (/^baz/ || [qw(foo bar)]) { ... }
will test only the regex, which causes both operands to be treated as boolean.
Watch out for this one, then, because an arrayref is always a true value, which
makes it effectively redundant.
Tautologous boolean operators are still going to be optimized away. Don't be
tempted to write
when ('foo' or 'bar') { ... }
This will optimize down to C, so C will never be considered (even
though the rules say to use a smart match on C). For an alternation like
this, an array ref will work, because this will instigate smart matching:
when ([qw(foo bar)] { ... }
This is somewhat equivalent to the C-style switch statement's fallthrough
functionality (not to be confused with I fallthrough functionality - see
below), wherein the same block is used for several C statements.
Another useful shortcut is that, if you use a literal array
or hash as the argument to C, it is turned into a
reference. So C is the same as C,
for example.
C behaves exactly like C, which is
to say that it always matches.
=head3 Breaking out
You can use the C keyword to break out of the enclosing
C block. Every C block is implicitly ended with
a C.
=head3 Fall-through
You can use the C keyword to fall through from one
case to the next:
given($foo) {
when (/x/) { say '$foo contains an x'; continue }
when (/y/) { say '$foo contains a y' }
default { say '$foo does not contain a y' }
}
=head3 Return value
When a C statement is also a valid expression (e.g.
when it's the last statement of a block), it evaluates to :
=over 4
=item *
an empty list as soon as an explicit C is encountered.
=item *
the value of the last evaluated expression of the successful
C/C clause, if there's one.
=item *
the value of the last evaluated expression of the C block if no
condition is true.
=back
In both last cases, the last expression is evaluated in the context that
was applied to the C block.
Note that, unlike C and C, failed C statements always
evaluate to an empty list.
my $price = do { given ($item) {
when ([ 'pear', 'apple' ]) { 1 }
break when 'vote'; # My vote cannot be bought
1e10 when /Mona Lisa/;
'unknown';
} };
Currently, C blocks can't always be used as proper expressions. This
may be addressed in a future version of perl.
=head3 Switching in a loop
Instead of using C, you can use a C loop.
For example, here's one way to count how many times a particular
string occurs in an array:
my $count = 0;
for (@array) {
when ("foo") { ++$count }
}
print "\@array contains $count copies of 'foo'\n";
At the end of all C blocks, there is an implicit C.
You can override that with an explicit C if you're only
interested in the first match.
This doesn't work if you explicitly specify a loop variable,
as in C. You have to use the default
variable C. (You can use C